When the insects appeared on Earth, they could not fly. One insect line out of the many
existing in the Paleozoic Era somehow evolved wings in the adult stage. The first species
with wings looked more like those classified today as paleoptera (Greek for
"old-winged"). For the first time, creatures appeared on Earth that did not have
to stay forever on the ground. The niche of flight was taken, and flying insects enjoyed
extensive diversification. Dragonflies and their kin reached enormous sizes at some point
in evolutionary time. Their characteristic is that they cannot fold their wings;
they keep them always extended. Descendants of the earlier, flightless insects are the
species belonging to the subclasses microcoryphia (jumping bristletails), and
thysanura (silverfish, etc.)

A little later, still in the Paleozoic Era, a lineage among the insects that could fly
evolved the ability to fold their wings back over the body when not in flight. This gave
them greater mobility on the ground, and an advantage in hiding and searching for food,
over other insects that had to keep their wings spread. This was the subclass
neoptera ("newly-winged"), which enjoyed further extensive
diversification, and resulted in most insect species with which we are familiar today.
If you descend to the subclass neoptera, you'll read what further evolutionary advances
occurred within that group.